Ergonomics, good configuration buttons. As against autonomy is a real scourge! For a power user like me I have to recharge every 2 to 3 days and it's long relaxation (rechargeable low cost!): A real purge (it's a ball this thread) !!! I prefer to feed with non-rechargeable alkaline batteries that increases the autonomy to +/- 15 days. In short, this is the mouse rolls and as such need to recharge the electric so what does wonders in the old fuel !!!